This is something that every business owner or would be entrepreneur thinks about at one point or another
When you're just starting out you need capital. Your goal should be just to cover your fixed costs while you make your business a viable proposition by creating a stream of profits. As the business grows you will be adding staff as these people cost less and they are productive, if you have been able to add them as fast as you wanted to get your business running. As your business expands you will be spending capital on other things such as marketing, expanding your product line and other expenses.
I recommend that you try to look at what your revenue will be first. This will give you a rough idea of how much capital you will need each month to cover your fixed costs. It will also give you a good idea of where your needs are covered and where you will need to spend money.
Once you have your revenue figure worked out you can work out how many hours will be required to make that revenue. Now you can see where you will be spending capital on other things. Of course this doesn't take into account the marketing you will need but it is a quick and easy way to figure out if your expenses are sustainable.
If you are just starting out you probably won't have a good handle on what will be a reasonable rate for your staff. You won't know how many hours they will work at what rate so you will need to do your own analysis. I recommend trying to hire by task and then look at what your costs are per hour. You will be able to see if you can afford the hours required.
The second thing that will affect the amount of money you have available to spend is the type of business. If the nature of your business is selling services you will probably only be able to spend a little bit on advertising for your service. I recommend trying to market your business by having your service offered by an independent person that can also provide some type of a service. This will also help limit the amount of capital that you need to spend each month buying additional advertising.